Gas display panels of the type to which this invention relates have two flat glass plates that are spaced apart and sealed to contain an ionizable medium. A set of horizontally extending insulated condutors (X drive lines) are located on one glass plate, and a set of vertically extending conductors (Y drive lines) are located on the other plate. These conductors are insulated by a dielectric layer. When a suitable voltage is applied between one horizontal conductor and one vertical conductor, ionization occurs in a region at the cross over point of the two conductors and light is emitted. The cross over points and ionized regions inbetween are called cells, and a display pattern is formed by ionizing selected cells. However, upon application of the firing voltage the cell ionizes and emits light only briefly as free charges formed by the ionization migrate to the insulating dielectric walls of the cell where these charges produce an opposing voltage to the applied voltage and thereby extinguish the ionization. The operation of initially ionizing a cell is called writing. Once a cell has been written a continuous sequence of light flashes can be produced by an alternating voltage called a "sustain" voltage. The amplitude of the sustain waveform can be made less than the amplitude required for the firing voltage, because the wall charges that remain from the preceding write or sustain operation produce a voltage that adds to the voltage of the sustain waveform to produce the ionizing voltage. A previously unwritten (or erased) cell is not ionized by the sustain waveform. In a gas panel of this type the sustain waveform is applied across all the horizontal conductors and all of the vertical conductors so that the gas panel maintains a previously written pattern of light emitting cells.
For a conventional write operation a suitable write voltage pulse is added to the sustain voltage waveform so that the combination of the write pulse and the sustain pulse produces ionization. In order to write an individual cell independently, each of the horizontal and vertical conductors has an individual selection circuit. Thus, applying a sustain waveform across all of the horizontal and vertical conductors but applying a write pulse across only one horizontal conductor and one vertical conductor will produce a write operation in only the one cell at the intersection of the selected horizontal and vertical conductors. An erase operation can be thought of as a write operation that proceeds only far enough to allow the previously charged cell walls to discharge; it is closely similar to the write operation except for timing and amplitude, and the circuits that produce both the write or erase pulses are called "write-erase circuits" or "selection circuits."
The selection circuit usually comprises a transistor switch for each horizontal conductor and each vertical conductor. The horizontal and vertical selection circuits may connect the associated conductors to the horizontal or vertical sustain waveform and to a selected one of the two voltage levels of a write-erase pulse.
In constructing a plasma display panel it is desirable to provide as many vertical and horizontal drive lines per linear inch as practicable. The resolution increases as the number of drive lines per linear inch increases. It is desirable to have high resolution since this permits characters to be drawn more precisely, thereby improving their definition. However, as the number of drive lines per linear inch is increased to a large number to provide greater resolution, the problem of selecting a given one of the horizontal drive lines or given one of the vertical drive lines becomes increasingly more difficult, particularly in large panels which may have a length of several feet or more. Thus, equipment for selecting and firing the numerous gas cells tends to become bulky and complex, and an ultimate limit is reached concerning the size of the display panel. It is to the objective of increasing resolution, increasing the size of a plasma display panel and at the same time providing selection equipment which is capable of performing the increased selection function without becoming unduly complex that the invention of the incorporated patent was directed. Essentially, the incorporated patent was to an invention which reduced the number of selection lines by a passive network external to the panel consisting of capacitance. The provision of the capacitance in the selection circuitry in that invention provided a means for reducing the number of selection lines, achieving a significant cost reduction and eliminating the difficult problems of connecting a large number of selection lines in a small amount of space to the X and Y drive lines.
As will be explained more fully hereinafter, a gas panel is essentially an analog device which can be represented by a capacitance. The ignition voltage must be capable of driving the panel capacitance and in the invention of the incorporated patent it must also drive the additional selection circuit capacitance external to the panel. To achieve proper operation, the external capacitance should be several times larger than the panel capacitance. Consequently, while the circuit of the incorporated patent worked very well for smaller panels, the load capacitance of large panels becomes so great that the requirements for external capacitance become impractical. Thus, it is an object of the present invention to retain the simplified selection circuitry of the incorporated patent and yet eliminate the need for external capacitance.